Cooking oils, such as those used in commercial or institutional deep fryers, tend to become contaminated with such things as moisture and carbonized food particles during frying. The oils also tend to break down chemically after extended use, often causing the oil to foam, smoke, smell bad, look bad, or taste bad. It has been found that keeping the cooking oil clean by removing the particulate matter and filtering it on a regular basis tends to extend the useful life of the cooking oil, and increases the quality and appearance of foods which are cooked therein.
Accordingly, a variety of specialized systems and filter media for filtering cooking oil have been developed. In most oil filtering systems, the contaminated oil is transferred from the deep fryer to a holding reservoir, pumped under vacuum through a filter medium, and then returned to the deep fryer for further use.
One widely used filter system and medium combination includes a vacuum chamber having a relatively flat upper surface in which a number of holes are formed. A thin piece of filter paper is laid over the upper surface of the vacuum chamber and held in place along its edges by a small number of stays. The vacuum chamber is coupled through an adaptor to the pumping unit of the main filter system. When in use the vacuum chamber, with filter paper intact, is placed at the bottom of the reservoir of contaminated oil. Through the action of the pumping unit, the contaminated oil is drawn through the paper filter, into the vacuum chamber, out through the adaptor, and returned to the deep fryer for reuse.
The vacuum chamber of this filter system has a seam extending around its perimeter where the top half and the bottom half of the chamber are joined. In use, contaminated oil can enter the vacuum chamber through this seam without passing through the paper filter, thereby being returned to the deep fryer in an unfiltered condition. This is particularly true when the upper surface of the paper filter becomes covered with debris, as the oil will naturally seek out a path of lesser resistance.
Additionally, this filter system allows for the passage of contaminated oil into the vacuum chamber around the edges of the filter paper. Because the design of this filter system relies upon the peripherally located stays and the pressure exerted by the vacuum chamber to hold the filter paper in place, a truly tight seal between the upper surface of the vacuum chamber and the filter paper cannot be maintained.
Another drawback of this design is that the holes in the upper surface of the vacuum chamber, through which the contaminated oil must pass, amount to only some fifty percent of the total surface area of the upper surface. Because contaminated oil can only pass through an area of the filter paper having an underlying hole, it is only approximately this same percentage of the filter paper that is utilized.
Another commercially available filter system and medium combination utilizes a vacuum chamber, but does not use a paper filter medium. Rather, a filtering powder (such as magnesium silicate) is added to the contaminated oil in the reservoir. The upper surface of the vacuum chamber contains a very fine mesh through which the contaminated oil is drawn. The filtering powder, which is suspended in the contaminated oil, begins to plate out over the fine mesh as the oil is drawn into the vacuum chamber. When a sufficient layer of filtering powder has so formed, a filter medium is created over the top of the fine mesh.
The disadvantages of this slurry-type filtering design are many. First, in the initial stages of filtering, the contaminated oil is not effectively filtered because a sufficient layer of filtering powder has not yet plated out. Thus, this design requires that the oil be run through two or more cycles in order to be sufficiently filtered. However, the pumping action of such recycling tends to damage the oil due to the oxidation, mechanical breakdown, and chemical breakdown of the large carbon chain compounds that make up the oil.
Another disadvantage of this design is that a portion of the filtering powder remains with the filtered oil that is returned for subsequent use, thereby potentially impairing the effectiveness of the filtered cooking oil and/or the taste of foods cooked therein. Additionally, this design requires that the vacuum chamber unit be broken down and the plated out layer of filtering powder be removed after each use.
With each of the two above-described designs, contaminated oil is drawn into the vacuum chamber through the upper surface of the chamber. Consequently, a residual amount of oil lying between the upper surface of the vacuum chamber and the lower surface of the reservoir never gets filtered or returned for subsequent use. This residual layer of oil is known as a "heel."
In a modified version of the first above-described design, a thicker cellulose-based filter pad, rather than a thin piece of filter paper, is used as the filter medium. Such systems experience drawbacks similar to those described above. Furthermore, such systems permit the filter pad to be improperly oriented during the filtering process. This improper orientation drawback is critical because many filter pads are formed with a mean pore size gradient through their thickness dimension. Namely, the mean pore size is greatest at and near the entrance surface of the filter pad and decreases to its smallest value at and near the exit surface. Gradation in pore size allows the larger contaminants to be first filtered near the entrance surface of the filter pad and the smaller contaminants to be subsequently filtered deeper within the filter pad. Thus, it is imperative that the filter pad be oriented so that the contaminated oil enters the proper surface of the filter pad.
As a result, there has been a long-felt need for a filter system and medium combination useful in filtering contaminated cooking oil that: (1) is relatively inexpensive to produce; (2) prevents contaminated oil from bypassing the filter medium and being returned for reuse in an unfiltered state; (3) fully utilizes the effective filtering area of the filter medium; (4) sufficiently filters the contaminated oil with a single pass through the combination; (5) causes no excessive damage to the oil during filtering; (6) leaves no residual heel; and (7) ensures proper orientation of the filter medium during the filtering process. This invention, when used in conjunction with various commerically available filter systems and a currently available filter medium, is directed to satisfying this need.